My miniature poodle loves picking everything up from the ground and I'm afraid one day he'll eat something that will cause poisioning. How can I make him vomit if this happens?
If your dog does, by chance, ingest something that may be poisonous, it's best not to try and get him to vomit. Instead take him directly to the vet.

Sometimes, by inducing vomitting, you can harm the dog's mouth, throat or stomach, which can then led to other health problems.
Usually when a dog gets intoxicated or poisoned you should see a vet right away.

I'd suggest that you train your dog to stop picking up every single thing from the ground. It's better to prevent troubles than to let your companion go through poisioning. You can never know how serious such a condition will be.

You could induce vomitting by giving your dog two teaspoons of Hydrogen Peroxide or a teaspoon of salt. But this should only be done as first aid, and showing your dog to a vet will be a must anyways.
Do NOT induce vomitting if your dog is having convulsions, seizures, shock or lethargy. Do NOT induce vomitting if your dog has swallowed caustic substances.